Title Crystallization of polyethylene under high pressure

Author(s) Osugi, Jiro; Hara, Kimihiko; Hirai, Nishio; Hikasa, Junichi

Citation The Review of Physical Chemistry of Japan (1965), 34(2): 59-64

Issue Date 1965-06-20

URL http://hdl.handle.net/2433/46850

Right

Type Departmental Bulletin Paper

Textversion publisher

Kyoto University

The Review of Physical Chemistry of Japan Vol. 34 No. 2 (1964)

THE REVIEW Ot PHYSICAL CH E'dISTRY OP JAPAN, VOL. 34, ;!IO. 2, 1964

CRYSTALLIZATION OF POLYETHYLENE UNDER HIGH PRESSURE

BY )IRO Osoal, KIafIHI%0 HARA, NISHID HIRAI" AND JliNICHI HI%ASA~

The crystallization and the annealing grocess of polyethylene under high pressure have been studied. The thickness of single crystal lamellae can be

represented well by the equation of Lauri[zen-Hoffman up to 2000 atm if one uses the melting temperature predicted by the Clapeyron equation. The surface free energy of the end surface of single crystal has been estimated,, and been found to decrease with increasing pressure.

Introduction

Till, Keller and Fischer found independentlyl:'lhat polyethylene crystallizes isothermally from the dilute solution as single crystal lamellae. After this discovery many polymers were crystallized in the form of single crystal, and now it is believed that any crystallizable polymers can b@ single crystals if proper solvents are chosen. The single crystal of polyethylene is a lamella of lozenge shape with neazly the same basic thickness whici is about 100A. It has been shown that the uniform thickness o[ plate-like crystals is associated with the fold Configuration of the chain mole-cules which are perpendiculaz [o the plane of the chain folds. The thickness of the lamella increases with the temperature of crystallization or annealing.

The experiment of the polymer crystallization under high pressure was done by \fatsuoka at first?7 He found that the linear polyethylene crystallized under high pressure has a density ap-proaching the perfect crystal density calculated from the crystal lattice constants obtained from x-ray analysis and its melting point i; close to the limiting value estimated for [he perfect crystals. Recently Geil and his coworkers3l found that the thickness of [he crystal lamella of linear polyethylene crystallized from melt under pressure of about 5000 atm becomes extremely lazge and sometimes it amounts to the length of the polymer molecule itself, that is, about several thousands angstrom.

The present work is cazried out to study the mechanism of the process of [he increase in the lamellar thickness under elevated pressure with rising temperature of crystallization.

ExperimentDls

(1) Preparation of samples The samples used in [his investigation were a linear polyethylene, Marlex 50. 0.2 g of Marlex

(Received, Jonuury IS, 1965) 1) P. A. Till, Jr., Polymv Sci., 24, 301 (1957)

A. Keller, Phil. 3faR•, 2, 1171 (1957) E. W. Fischer, Z. NarurJonch. I2a, 753 (1957)

2) S. Dfatsuoka, J. Polymer Sci., 42, 511 (1960) 3) P. H, Geil, F. R. Anderson, B. Wunderlich and T. Arakawa, !. Polymer Sci., Part A 2, 3702 (1961)

* Deportment of Ckerois[ry, Facufly of Science, Okaynnro C'niversily, Okayavra, laqun

i

i iThe Review of Physical Chemistry of Japan Vol. 34 No. 2 (1964)

i

60 J. Osugi, li. Hara, !V. Hirai and J. Hikasa

i0 was dissolved in 200 cc of xylene at 120°C. This solution was gradually added into xylene , whose volume was about 5 times as much as that of the solution, when the temperature was kept constant

within I`C above a given crystallization temperture. The suspension was aged isothermally for a

few hours before filtration. After filtration and drying the thin film of about 0.05 mm thick was

taken ofi from the filter paper. The single crystals were piled up in the 51m orienting their lamellar

surface parallel to the film surface. The thin film was cut into small pieces o[ ribbon with a razor.

(2) Hea[ treatment under high pressure

The high pressure apparatus used is the piston cylinder type vessel with silicon oil as pressure

transmitting medium, which produces the hydrostatic pressure (Fig. 1). The pressure and tempera-

ture were kept constant for 3 hrs to accomplish the thickening process. Then the temperature was

reduced to about 50°C at a rate of about 4°C/min at which time the pressure was released and the

sample removed. The small ribbons of the sample e•ere charged in parallel in a sample holder. and

submitted to the small angle x-ra}• measurement at room temperature. In order to obtain a sharp

diffraction pattern as shown in Fig. 2, the x-ray beam should be directed parallel to the surface of

I

~,

~~

~~

s,--

I

]BD'C

32°C1

AO'

121'C

~130'C

zo w so ao ao m to - _ 2B

Fig. 1 Diffraction intensity curve of low angle a-ray for single crystal film of Marlea-50 annealed

Fig. t The hydraulic press and piston cylin- at various temperatures under t,020 atm for der type high pressure apparatus 3 hrs

the film in which the single crystals were piled up with their flat surface parellel to the film surface.

(3) Crystallization from melt under high pressure Pellets of 14arlex 50 were enclosed in the above vessel under the chosen pressure and heated

up to about 2i0°C to melt the polymer. Then it is reduced gradually down to l80`C and kept at

the temperature and pressure for 3 hrs to accomplish the crystallization. After removal of the sam-

ples from the crystallization vessel, they were immersed in liquid nitrogen (or a sufficient period of time to cool them and were then fractured by a hammer. The fractured surfaces were replicated by

a dire[ platinum carbon technique, and the replicas were eaamined by means of an electron

microscope.

Crystallization

The Review of Physical Chemistry of Japan Vol. 34

of Polyethylene under High Pressure

No.2 (1964)

61

Results

(1) Heat treatment under high pressure The thickness of the single crystals of po]yetSylene annealed at atmospheric pressure increased

with the rise of annealing temperature as shown in Fig. 3. A single crystal heated for 1.5 hr a[ 120°C

at atmospheric pressure is shown in Photo. 1. Comparison between the area of the annealed crystal

i

a~

B ._

m c

s~

]m

O

~a~„

1031 atm 1970 atm

1~

i

Fig.

tm tm tw rm

T, °C

3 Thickness of single crystal lamela as a function

of annealing temptralure at various pressures

and that

thickened

of the original

about 2 times.

lozenge

Small

shape

angle

Pho[o. ]

printed

x-ray

on the collodion

diffraction for the

2aa

B .~ Pi m

3 uo

membrane shosss

sample annealed

that the former

under the same

Electron micragraph of a

single crystal of DSarle: 50

heated at 120°C for 2 hrs

under atmospheric pressure

~1 avn. 110'C

1970atm, len'C

Fig

]m

. 4 Increase in thic

single crystals

Annealing time, min

kness of hlarlex 50

during annealing

The Review of Physical Chemistry of Japan Vol. 34 No. 2 (1964)

62 J° Ogugi, K. Hara, 1\. Hirai and J, Hikasa

condition showed that the thickness of the crystallites wac about 2209. The morphological features of the single crystal annealed under high pressure would be the same as those at [he atmospheric

pressure. The thickness of the single crystals of polyethylene treated under va•ious pressures is shown in Fig. 3. It is shown that the annealing temperature T(°C) and pressure P (atm) have a

pronounced influence on the thickness L (A). The values of L increase with T slowly in the lower temperature range and steeply in the higher temperature range near the melting points for each

pressure. The annealing time does not affect the thickening process so much when it is above about 2 hrs as shown in Fig. 4.

(2) Electron microscope observation The (ratlure surface of [he sample crystallized at 160°C under 1100 atm is shown in Photo. 2.

In the large portion of [hz photograph, the band structure of fairly uniform thickness can be observed.

This structure represents the lamellae which are seen edge-on. The polymer molecules are folded

in order to fit into the band structure and the molecular chain aces arz essentially perpendicular

to the lamellar thickness. The striations of [he order of 3009 in width normal to the band

structure would not be the molecule itself, but might be attributed to the molecular chain bandies.

They are, however, beneath the rzsolving power of the replicas.

The thickness of lamellae is in the order of 300 [o lOOOA. The whole length of the moleculaz

chain of ~fadex 50 is about SOODA, and so the molecules are extremely extended as long as 10009

but not fully exteoded and folded in these bands. It is also interest to notice that the fold length of

!0009 is consistent with the value expected from the lamellar thickness of single crystals annealed

under high pressure. The point (L=10009 and T=ItiO°C) is almost on the extrapolated straight

line of L us° 1 /(T„-T) plot for P=1 t00 atm as shown in Fig. 5.

The dark 5brous matters of about 3009 in width which are seen in the whole range and pre-

dominantly in [he lower righthand side of Photo. 2 may be the fibers in which the molecules are

pulled out from the folded-chain lamellae. Pulling-out was carried out so rapidly that the energy

produced was dissipated as heat. This heating causes the fibrils [o melt at their ends and ball up in the manner seen in Photo. 2.

I _.~ uc, r '~' J z

~Y® '~ 3'~a ~,

`° ~ ~ 1 a ~ ' ~ ~~

Photo. 2 Fracture surface of poly- ethylene (unfractionated

bfarlea 50)crystallized at 160°C under 1,125 a[m

taoa

soo

•a '8 .~ 8

s

10a~

p=1.fm

1870um

~oza am

Fig. 5

02 p,{ 0.8

I/(T.-T) Plot of L against 1/(Tm T) for hfarles 50 crystallized at various pressures

The Review of Physical Chemistry of Japan Vol. 34 No. 2 (1964)

Crystallization of Polyethylene under Aigh Pressure 63

Consideration

According to the theory of Lauritzen-Ho[fman?> L, [he thickness of the lamellae of the single

Crystals crystallized from melt at the temperature T, is given by Che following equation.

d,o, d j, Here, d~ is the diameter of the chain molecule, o, and o~ the surface free energies per unit area of

the lateral and the end surfaces of lamellae respectively, and dj the free energy of crystallization

which is represented by

where dk is the heat o[ fusion per unit volume of the polymer. Thus eq. (1) is deduced to the sim-

ple equation

L=L~+T „A T with A=2kT/d,o, and L,=kT/d,a„ (3 )

in which L, is the value of L when T is comparably close to T,,. The data of the lamellar thickness

L in Fig. 3 are replotted against t/(T.,„-T) in Fig. S. We have good straight lines for each pressure

in these temperature range as eq. (3) requires. The values of T„ are decided by the asymptotes of

each curve in Fig. 3. This result implies that the crystallization process under elevated pressure

seems to be ruled by the mechanism of Lauritun~Hoffman as in the case of the atmospheric pres-

sure. From the intercept and the inclination of the straight line, the values of La and A, and accord-

ingly o, were calcuated, which zee listed in Table 1.

Table 1

P(atm) L,(Apr A(A/deg.) T„('C) o.(erg/cmr)

1

1020

I9i0

96

90

83

zzoo

1850

1350

140

161

I85

~~

62

43

• Calculated values a[ T,„-T-50

The melting point T,„ should be elevated with increasing pressure P and may be predicted from

the Clapeyron equation ;

dP dH (4) dT,,,-T,udV'

where dH (cal/g) denotes the latent beat of fusion, T (°K) the melting point, and dV (cc/g) the

volume change of fusion. At least in the pressure range of the present experiment, dH and dV may

be assumed to be constant, and eq. (4) can be approximated by

P_dA1nT„,T dH T„-T,.), (5) dV T'„iTwdV( r

4) J. D. Hoffman and J. I. Lauritzen, Ja, !. Re.c. N. d. S., 65A, 197 (1961)

The Review of Physical Chemistry of Japan Vol. 34 No. 2 (1964)

64

where T ,' is the

dV=0.16cc/g and

E m

a:

The values

and theref

The v

order as t

The value

considered

where 700

for a fold

is 60 erg/t

listed in T

Eq. (3

the reverse

of the sing

doubtful

assume tha

high press

J. Osugi, IC. Hara, N. Hirai and J. Hikasa

is the melting temperature at the atmospheric pressure. Now put dH=6g.Ocal/g,

c/g and T',„=410°K, then we heve

P(alm)=425(T.,-410). (6 )

0

Fig. 6 Effect of pressure (P) on the melting point (T,q) of polyethylene; (O)

experimental points. The solid line shows eq. (6).

tso tso r7o tao tso Tom. `G

of P and T,„ listed in Table 1 are in accord well with this equation as shown in Fig. 6,

ore [hey are considered to be adequate as the melting temperatures for the given pressures.

aloes of a, calculated from eq. (3) are less than 10 erg/cmz which are almost the same he surfaze free energy of liquid paraffins, but the values of a° calculated are much larger.

of a, can be estimated from the molecular conformation of the chain folding which is

to contain four gauche configurations is it. The excess energy for a fold is 700 x 4 caL,

cal. is the energy difference between [he [rans and the gauche forms. Since the surface area

is equal to 36.3 x 10-'s cmz, the excess surface energy per unit area of the end surface

mz, and the value of a. be=omes 60+10=70 erg/cmz which agrees well with the valves

able 1.

shows that A u proportional to T. and so ft is expected to decrease with pressure, but

is true as seen is Table 1. This means that the surfaze free energy of the end surface le crystal a,: would decrease with increasing pressure as is represented in Table 1. It is

whether any quantitative treatment of this fact is possible as yet, but it is reasonable to

t the energy difference between the gauche and [rans configurations becomes less under

ore. Laboratory of Physical Chemistry

Department of Chemistry

Faculty of Scie>ue

Kyoto University Kyoto, Japan